The importance of lithium secondary batteries, nickel hydride batteries, and other secondary batteries as vehicle-mounted power sources and as power sources for personal computers and portable and mobile electronic devices has been on the rise in recent years. In particular, lithium secondary batteries, which are light and provide a high energy density, are expected to be preferably used as high-output vehicular power sources. A typical configuration for this type of lithium secondary battery is provided with an electrode assembly having a structure in which sheet-shaped electrodes are wound into a spiral shape (wound electrode assembly). To form this wound electrode assembly, for example, a positive electrode sheet having a structure in which a positive electrode active material layer containing a positive electrode active material is supported on both sides of a positive electrode current collector and a negative electrode sheet having a structure in which a negative electrode active material layer containing a negative electrode active material is supported on both sides of a negative electrode current collector are wound into a spiral shape with a separator sheet interposed therebetween. In order to ensure ionic permeability between the positive electrode and negative electrode, this separator sheet takes the form of a resin sheet of, e.g., polyethylene (PE), polypropylene (PP), and so forth, in which a large number of pores are formed.
Separator sheets of, e.g., polyethylene (PE), polypropylene (PP), and so forth, do offer the advantage of exhibiting a shutdown function at suitable temperatures (for example, approximately 130° C.); however, due to their oxidative deterioration when exposed to the charging potential of the positive electrode, they are also associated with the problem of a decline in battery performance. Furthermore, they change shape upon melting at temperatures of 140° C. to 150° C., and as a consequence, when the battery temperature rises further even after shutdown, the separator sheet undergoes a change in shape (heat shrinkage), creating the possibility for an internal short circuit to occur.
The placement of a porous, heat-resistant filler layer on the surface of the separator sheet has been investigated for the purpose of preventing the short circuiting brought about by heat shrinkage of the separator sheet. For example, Patent Literature 1 describes an art in which a porous heat-resistant layer (filler layer) containing a finer made of a metal oxide is disposed on the side where the positive electrode plate and negative electrode plate face each other. Other art relative to this type of heat-resistant filler layer is provided in, for example, Patent Literature 2 and Patent literature 3.